Method for Producing Polishing Bar Made of Valve Steel 53Cr21Mn9Ni4N

ABSTRACT

This invention provides a method for producing a polishing bar made of valve steel 53Cr21Mn9Ni4N. The process flow is as below: electric furnace smelting→outside-the-furnace refining→continuous casting→heating→high-speed wire rolling→heat treatment (off-line/online)→straightening→polishing→inspection→storage. The production method of the invention adopts a casting method of continuous casting to overcome the poor high-temperature plasticity and the great deformation resistance of 53Cr21Mn9Ni4N which leads to a difficulty in continuous casting. The continuous casting is directly carried out on casting billets for rolling, which greatly reduces the material loss and energy consumption of this step, and improves the yield of the billets. The high-speed wire precision rolling technology is directly used on the continuous casting billets to improve the production efficiency, and greatly improve the size precision control of rolled and finished wire rods.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims priority to Chinese Patent Application No. CN2016107229472, filed on Aug. 25, 2016, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The invention relates to the field of iron and steel smelting technology, specifically relates to a method for producing a polishing bar made of valve steel 53Cr21Mn9Ni4N.

BACKGROUND OF THE INVENTION

Valve steel 53Cr21Mn9Ni4N is a kind of austenitic valve steel in accordance with the Chinese standard GB/T12773-2008, referred to as 21-4N, comprising the following components in weight percent: C: 0.48˜0.58%; Si: ≦0.35%; Mn: 8.00˜10.00%; P: ≦0.040%; S≦0.030%; Ni: 3.25˜4.50%; Cr: 20.00˜22.00%; N: 0.35˜0.50%; Cu: ≦0.30%; C+N≧0.90%.

53Cr21Mn9Ni4N steel is an austenitic heat resistant steel with carbide as a precipitated hardening phase. This steel has a poor thermal conductivity, a large thermal expansion coefficient and a large deformation resistance, and the plasticity thereof is worse than that of the general austenitic heat resistant steel. The 53Cr21Mn9Ni4N steel is an austenitic aging steel, and is also often classified as a hard deformation steel, which is difficult to be produced. Because of its high contents of manganese, nickel, carbon, nitrogen and chromium, the 53Cr21Mn9Ni4N steel has an austenitic microstructure and a high crystallization temperature. Thus, it has high strength at high-temperatures, and improves the heat-resistant and anti-corrosive properties. High contents of manganese and chromium provide the steel with the austenitic structures at room temperature. The high contents of carbon and nitrogen result in a strong precipitation hardening effect, and enhance strength, hardness and wear resistance. The steel is generally used in a medium speed, high power, medium load engine working at a temperature of 850° C., and is widely applied in both domestic and foreign car exhaust valves.

Because of its characteristics of the 53Cr21Mn9Ni4N valve steel, the production process of a metal bar is usually as follows: electric furnace smelting→outside-the-furnace refining→casting→grinding→heating→cogging (rolling cogging or forging cogging)→decortication or grinding treatment→heating→rolling→heat treatment→straightening→polishing→inspection→storage. The product quality requirements of 53Cr21Mn9Ni4N are strict. The complicated production process and the applications of rolling or forging cogging lead to a large heating and grinding loss and a frequent phenomenon of cracking, resulting in a low yield. The cracking that occurs during heat treatment, has always been a focus and difficult problem of the manufacturering industry. For valve steel of a small size, the control of size tolerance and dimensional accuracy is more difficult. The above factors directly lead to the low yield of valve steel 53Cr21Mn9Ni4N, which affects the economic benefits of production enterprises.

SUMMARY OF THE INVENTION

In view of the above, it is an objective of the present invention to provide a new production process of the valve steel 53Cr21Mn9Ni4N, so as to improve a production efficiency and precision control of the wire rod size, and overcome the poor high-temperature plasticity and the large deformation resistance of the 53Cr21Mn9Ni4N which leads to difficulty in continuous casting.

In order to achieve the above objective, the method for producing a polishing bar made of valve steel 53Cr21Mn9Ni4N comprises the following steps:

S1. providing scrap steel and alloys including high carbon ferrochrome and ferronickel into an electric-arc furnace for melting and primary smelting, wherein a tapping temperature is higher than 1600° C. and P is less than 0.040%; S2. AOD smelting, blowing nitrogen to smelt with oxygen in an oxidizing period, when a temperature reaches over 1660° C. and C is less than 0.60%, starting to add a predefined amount of ferrosilicon for a reduction operation, blowing nitrogen for alloying, adjusting alloy composition; tapping steel liquid when the ingredients meet a requirement of standard and the temperature is higher than 1490° C., and transferring the steel liquid to a ladle furnace for refining; S3. ladle furnace refining, fine tuning a composition and the temperature of the steel liquid, maintaining a white slag operation, tapping the steel liquid when the temperature of liquid steel reaches a temperature of continuous casting and packaging; S4. continuously casting through a billet continuous casting machine, wherein a continuous casting and packaging temperature is as below: first continuous casting packaging at 1490˜1510° C., continuous packaging at 1480˜1500; controlling superheat of the steel liquid between 25˜35° C. during pouring, using a combination of crystallizer and final electromagnetic stirring when pouring; marking continuous casting billets when ejecting the billets and air-cooled stacking; S5. heating before rolling, heating in a pusher-type heating furnace or a walking beam type heating furnace; S6. high-speed wire rolling at a rolling temperature of 1160˜1190° C., controlling a finish rolling and spinning temperature over 1050° C., force cooling by water spray after spinning; entering into a collection roll after cooling, packaging after a collection and transporting a wire rod into a heat treatment process; S7. heat treating the wire rod; S8. straightening, immersing the wire rod into a lime coating solution and maintaining for a while after a heat treatment of 53Cr21Mn9Ni4N wire rod, pulling out the wire rod, drying the wire rod naturally until a surface coating of the wire rod becomes dry and white; and straightening the wire rod using a seven-roll bar straightener at a straightness less than 1 mm/m; and S9. polishing, rough grinding first and then fine grinding to meet design requirements using a centerless grinder for processing.

Preferably, in the step 4, the temperature during pouring is 1415-1430° C., and a casting speed is 1.3-1.0m/min.

Further preferably, in the step 4, a cooling control mode of a secondary cooling area is provided with three-phase partitioning water distribution, when the casting speed is less than 1.0m/min, three phases of water in the secondary cooling area are all opened, a ratio of water distribution of the three phases is 50%:30%:20%; a water ratio automatically matches with the casting speed; when the casting speed is within a normal range of 1.0˜1.2m/min, only first two phases of water in the secondary cooling area are opened and the ratio of water distribution is 55%:45%, a total water ratio is controlled to be 0.27˜0.32 L/Kg steel.

Further preferably, in the step 5, a heating temperature of the continuous casting billets is controlled as below: the heating temperature in a preheating section is lower than 1050° C.; the heating temperature in a heating section is 1030˜1080° C., the heating temperature in a soaking section is 1170˜1210° C.

Further preferably, in the step 6, a wire rolling speed is controlled as below: the wire rolling speed at an exit section of intermediate rolling is 1.10˜1.15 m/s, the wire rolling speed at an exit section of pre-finish rolling is 4.78˜5.64 m/s, the wire rolling speed at an exit section of finish rolling is 28˜33 m/s.

Further preferably, the heat treatment in the step 7 includes an off-line heat treatment and an online heat treatment. wherein the off-line heat treatment includes the following steps: solution treating a 53Cr21Mn9Ni4N hot rolling disk at a solution temperature of 1040-1150° C. for 30-60 min; wherein a cooling is performed in a water-cooled mode; a grain size is required to be finer than 6 grade and a face hardness of a sample is 32˜37 HRC; wherein the online heat treatment includes the following steps: rolling 53Cr21Mn9Ni4N continuous casting billets through a high speed mill, wherein a heating temperature of rolling meets requirements described in the step 5, the wire rod is performed with the online heat treatment after spinning, and put into water for cooling, wherein the grain size is required to be finer than 8 grade, and the face hardness of the sample is 31˜36 HRC.

Compared with the existing production process, this invention has the following advantages:

-   -   (1) The casting method of continuous casting overcomes the poor         high-temperature plasticity and great deformation resistance of         53Cr21Mn9Ni4N which leads to a difficulty in continuous casting.         The steps of “casting→grinding→cogging (rolling cogging or         forging cogging)→decortication or grinding treatment→heating” in         the original production is replaced by directly rolling the         continuous casting billets, which greatly reduces the material         loss and energy consumption, and improves the yield of the         billets.     -   (2) The high-speed wire precise rolling technology is directly         used on the continuous casting billets to improve the production         efficiency, and greatly improve the size precision control of         the rolled and finished wire rods. Thus, the “one-heating         forming” is achieved to directly roll the continuous casting         billets of 53Cr21Mn9Ni4N steel to the wire rods.     -   (3) Two modes of the heat treatment, i.e., off-line or online,         are realized for a free choice of the customer according to the         quality requirements.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objectives, features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings. Wherein:

FIG. 1 is a flow chart showing a method for producing a polishing bar made of valve steel 53Cr21Mn9Ni4N in the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a flow chart showing the method for producing valve steel 53Cr21Mn9Ni4N polishing bar of the present invention. The production technical process is summarized as follows: electric furnace smelting→outside-the-furnace refining→continuous casting→heating→high-speed wire rolling→heat treatment (off-line/online)→straightening→polishing→inspection→storage. The method includes a whole production process of the valve steel 53Cr21Mn9Ni4N, including smelting, continuous casting, high-speed wire rod rolling, heat treatment, and producing a finished product. The production method of the present invention is described in detail as below, with reference to specific embodiments.

Embodiment 1

A furnace of 53Cr21Mn9Ni4N wire rods with a specification of φ6.7 mm is produced. The production technical process is as follows: 25t electric furnace smelting→25t AOD smelting→25t LF refining→Two-flow continuous casting for billets having a cross section of 150×150 mm→heating→high-speed wire rolling→off-line heat treatment→straightening→polishing→inspection→storage. The specific production steps are as follows:

Step 1: Providing scrap steel and alloys such as high carbon ferrochrome, ferronickel, etc. into an electric-arc furnace for melting and primary smelting. The tapping temperature is 1620° C., P: 0.030%; Step 2: AOD smelting, blowing nitrogen to smelt with oxygen in an oxidizing period, when the temperature reaches ≧1660° C. and C≦0.60%, starting to add a certain amount of ferrosilicon for a reduction operation, blowing nitrogen for alloying, adjusting alloy composition. Tapping the steel liquid when the ingredients meet the requirement of standard and the temperature ≧1490° C., and then transferring the steel liquid to LF (LADLE FURNACE) for refining; Step 3: LF refining, fine tuning composition and temperature, maintaining a white slag operation, tapping the steel liquid when a temperature of liquid steel reaches the temperature of continuous casting and packaging. The composition of the liquid steel is: C: 0.54%; Si: 0.31%; Mn: 8.53%; P: 0.028%; S: 0.007%; Ni: 3.76%; Cr: 21.80%; N: 0.39%; soft blowing and letting stand for 10 min after tapping the liquid steel, and then continuous casting and packaging; Step 4: The temperature of continuous casting and packaging is 1507° C. A combination of crystallizer and final electromagnetic stirring is used when pouring, wherein target parameters of the crystallizer electromagnetic stirring are as below: I (electric current)=380 A, f (frequency)=˜3.5 Hz; target parameters of the final electromagnetic stirring are as below: I=350 A, f=7˜8 Hz; two-flow casting at a start pulling speed of 0.5m/min, turning to a liquid automatic control system after casting normally for 2 min, slowly raising the pulling speed to 1.0 m/min when the liquid level stays steady, paying attention to the temperature drop situation of packaging during pouring, increasing the pulling speed to 1.2 m/min when the superheat is between 25˜35° C., pulling quickly at a low temperature to ensure the quality of continuous casting billets, sizing the continuous casting billets at 3 meters, and air-cooled stacking; Step 5: Heating before rolling, heating in a pusher-type heating furnace, and controlling the heating temperature of the continuous casting billets as below: preheating section ≦1050° C.; heating section: 1050˜1080° C., soaking section: 1180˜1200° C.; Step 6: High-speed wire rolling, at an initial rolling temperature of 1170, the temperature of entering into a finishing mill is 1030° C. and the finish rolling and spinning temperature is 1120° C., and cooling in water after spinning.

A rolling speed at the exit section of intermediate rolling is 1.15 m/s. A speed at the exit section of pre-finish rolling is 5.60 m/s. A rolling speed at the exit section of finish rolling is 33 m/s. The specification is φ6.7 mm±0.15 mm. Spinning and force cooling by water; cooling through Steyr Moore air-cooled line and then entering into collection rolls, packaging after collection, and transporting the wire rods and entering into heat treatment process;

Step 7: Heat treatment, the off-line solution treatment is used at a solid solution temperature of 1060-1080° C. for 50 min. Cooling method: water cooling. Grain size: 8-10 grade, face hardness of the sample: 32˜35 HRC; Step 8: Straightening, immersing the wire rods into a lime coating solution and maintaining for a while, pulling out the wire rods, drying the wire rods until the surface coating of the wire rods becomes dry and white; straightening the wire rods using a seven-roll bar straightener at a straightness ≦1 mm/m; Step 9: Polishing, rough grinding three times and then fine grinding for one time, using a M1080 centerless grinder, to obtain polishing bars with a specification of φ6.7 mm. The mechanical properties of the polishing bars after heat treatments (solid solution+aging treatment) are as follows:

Serial Stipulated Elongation Shrinkage number Tensile non-proportional after of cross- Hard- of Strength extension strength fracture section ness samples (R_(M)/MPa) (R_(P0.2)/MPa) (A/%) (Z/%) (HRC) 1# 1080 750 11 14 30.0 2# 1060 730 12 15 29.5

Embodiment 2

A furnace of 53Cr21Mn9Ni4N wire rods with a specification of φ6.5 mm is produced. The production technical process is as follows: EBT electric furnace smelting→AOD smelting→LF refining→Two-flow continuous casting for billets having a cross section of 150×150 mm→heating→high-speed wire rolling→online heat treatment→straightening→polishing→inspection→storage. The specific production steps are as follows:

Step 1: Providing scrap steel and alloys such as high carbon ferrochrome ferronickel, etc. into an electric-arc furnace for melting and primary smelting. The tapping temperature is 1610° C., P: 0.036%; Step 2: AOD smelting, blowing nitrogen to smelt with oxygen in an oxidizing period, when the temperature reaches ≧1660° C. and C≦0.60%, starting to add a certain amount of ferrosilicon for a reduction operation, blowing nitrogen for alloying, adjusting alloy composition. Tapping the liquid steel when the ingredients meet the requirement of standard and the temperature ≧1490° C., and then transferring the liquid steel to LF for refining; Step 3: LF refining, fine tuning composition and temperature, maintaining a white slag operation, tapping the liquid steel when a temperature of liquid steel reaches temperature of continuous casting and packaging. The composition of the liquid steel is: C: 0.53%; Si: 0.25%; Mn: 8.67%; P: 0.032%; S: 0.005%; Ni: 3.83%; Cr: 21.05%; N: 0.41%; soft blowing and letting stand for 15 min after tapping the liquid steel, and then continuous casting and packaging; Step 4: The temperature of continuous casting and packaging is 1500° C. When pouring, the target parameters of crystallizer electromagnetic stirring are as below: I=380A, f=˜3.5 Hz, and the target parameter of the final electromagnetic stirring are as below: I=350A, f=7˜8 Hz; start pulling at a speed of 0.6m/min, turning to a liquid automatic control system after casting normally for 3 min, slowly raising the pulling speed to 1.1 m/min when the liquid level stays steady, paying attention to the temperature drop situation of packaging during pouring, increasing the pulling speed to 1.2 m/min when the superheat is between 25˜30° C., pulling quickly at a low temperature to ensure the quality of continuous casting billets, sizing the continuous casting billets at 3 meters, and air-cooled stacking; Step 5: Heating before rolling, heating in a walking beam type heating furnace, and controlling the heating temperature of the continuous casting billets as: preheating section ≦1050° C.; heating section: 1030˜1060° C., soaking section: 1170˜1190° C.; Step 6: High-speed wire rolling, at an initial rolling temperature: 1180° C., the finish rolling and spinning temperature is 1110° C., and cooling in water after spinning. A rolling speed at the exit section of intermediate rolling is 1.10 m/s. A speed at the exit section of pre-finish rolling is 5.30 m/s. A rolling speed of finish rolling at the exit section is 28 m/s. The specification is φ6.5 mm±0.15 mm. Spinning and cooling in water; cooling through Steyr Moore air-cooled line and then entering into collection rolls, packaging after collection, and transporting wire rods and entering into heat treatment process; Step 7: Cooling in water online. The grain size: 10-12 grade, face hardness of the sample: 34˜37 HRC; Step 8: Straightening, immersing the wire rods into a lime coating solution and maintaining for a while, pulling out the wire rods, drying the wire rods naturally until the surface coating of the wire rods becomes dry and white; straightening the wire rods using a seven-roll bar straightener at a straightness ≦1 mm/m; Step 9: Polishing, using a M1080 centerless grinder, to finally obtain polishing bars with a specification of φ6.5 mm.

The mechanical properties of the polishing bars after heat treatments (solid solution+aging treatment) are as follows:

Serial Stipulated Elongation Shrinkage number Tensile non-proportional after of cross- Hard- of Strength extension strength fracture section ness samples (R_(M)/MPa) (R_(P0.2)/MPa) (A/%) (Z/%) (HRC) 1# 1070 760 18 20 35.0 2# 1100 765 14 16 36.5

The method for producing a polishing bar made of valve steel 53Cr21Mn9Ni4N in this invention uses the casting method of continuous casting to overcome the poor high-temperature plasticity and great deformation resistance of 53Cr21Mn9Ni4N which leads to a difficulty in continuous casting. The steps of “casting→grinding→cogging (rolling cogging or forging cogging)→decortication or grinding treatment→heating” in the original production is replaced by directly rolling the continuous casting billets, which greatly reduces the material loss and energy consumption, and improves the yield of the billets. The conventional production uses the rolling cogging or forging cogging and requires heat treatment of the billets before cogging and rolling. It's commonly known as “two-heating forming”. In this invention, the high-speed wire precise rolling technology is directly used on the continuous casting billets to improve the production efficiency, and greatly improve the size precision control of the rolled and finished wire rods. Thus, the “one-heating forming” is achieved to directly roll the continuous casting billets of 53Cr21Mn9Ni4N steel to the wire rods. The heat treatment can be achieved in two ways, off-line or online, and the method can be chosen by the customer according to the quality requirements.

The present invention is not limited to the described embodiments, and those skilled in the art can still make corrections or alterations without departing from the spirit and scope of the invention, and the scope of the present invention is decided by the scope defined in the claims. 

What is claimed is:
 1. A method for producing a polishing bar made of valve steel 53Cr21Mn9Ni4N, comprising the following steps: S1. providing scrap steel and alloys including high carbon ferrochrome and ferronickel into an electric-arc furnace for melting and primary smelting, wherein a tapping temperature is higher than 1600° C. and P is less than 0.040%; S2. AOD smelting, blowing nitrogen to smelt with oxygen in an oxidizing period, when a temperature reaches over 1660° C. and C is less than 0.60%, starting to add a predefined amount of ferrosilicon for a reduction operation, blowing nitrogen for alloying, adjusting alloy composition; tapping steel liquid when the ingredients meet a requirement of standard and the temperature is higher than 1490° C., and transferring the steel liquid to a ladle furnace for refining; S3. ladle furnace refining, fine tuning a composition and the temperature of the steel liquid, maintaining a white slag operation, tapping the steel liquid when the temperature of liquid steel reaches a temperature of continuous casting and packaging; S4. continuously casting through a billet continuous casting machine, wherein a continuous casting and packaging temperature is as below: first continuous casting packaging at 1490 ˜1510° C., continuous packaging at 1480 ˜1500; controlling superheat of the steel liquid between 25−35° C. during pouring, using a combination of crystallizer electromagnetic stirring and final electromagnetic stirring when pouring; marking continuous casting billets when ejecting the billets and air-cooled stacking; S5. heating before rolling, heating in a pusher-type heating furnace or a walking beam type heating furnace; S6. high-speed wire rolling at a rolling temperature of 1160˜1190° C., controlling a finish rolling and spinning temperature over 1050° C., force cooling by water spray after spinning; entering into a collection roll after cooling, packaging after a collection and transporting a wire rod into a heat treatment process; S7. heat treating the wire rod; S8. straightening, immersing the wire rod into a lime coating solution and maintaining for a while after a heat treatment of 53Cr21Mn9Ni4N wire rod, pulling out the wire rod, drying the wire rod naturally until a surface coating of the wire rod becomes dry and white; and straightening the wire rod using a seven-roll bar straightener at a straightness less than 1 mm/m; and S9. polishing, rough grinding first and then fine grinding to meet design requirements using a centerless grinder for processing.
 2. The method for producing a polishing bar made of valve steel 53Cr21Mn9Ni4N of claim 1, wherein in the step 4, the temperature during pouring is 1415-1430° C., and a casting speed is 1.3-1.0m/min.
 3. The method for producing a polishing bar made of valve steel 53Cr21Mn9Ni4N of claim 2, wherein in the step 4, a cooling control mode of a secondary cooling area is provided with three-phase partitioning water distribution, when the casting speed is less than 1.0m/min, three phases of water in the secondary cooling area are all opened, a ratio of water distribution of the three phases is 50%:30%:20%; a water ratio automatically matches with the casting speed; when the casting speed is within a normal range of 1.0˜1.2 m/min, only first two phases of water in the secondary cooling area are opened and the ratio of water distribution is 55%:45%, a total water ratio is controlled to be 0.27˜0.32 L/Kg steel.
 4. The method for producing a polishing bar made of valve steel 53Cr21Mn9Ni4N of claim 1, wherein in the step 5, a heating temperature of the continuous casting billets is controlled as below: the heating temperature in a preheating section is lower than 1050° C.; the heating temperature in a heating section is 1030˜1080° C., the heating temperature in a soaking section is 1170˜1210° C.
 5. The method for producing a polishing bar made of valve steel 53Cr21Mn9Ni4N of claim 1, wherein in the step 6, a wire rolling speed is controlled as below: the wire rolling speed at an exit section of intermediate rolling is 1.10˜1.15 m/s, the wire rolling speed at an exit section of pre-finish rolling is 4.78˜5.64 m/s, the wire rolling speed at an exit section of finish rolling is 28˜33 m/s.
 6. The method for producing a polishing bar made of valve steel 53Cr21Mn9Ni4N of claim 1, wherein the heat treatment in the step 7 includes an off-line heat treatment and an online heat treatment, wherein the off-line heat treatment includes the following steps: solution treating a 53Cr21Mn9Ni4N hot rolling disk at a solution temperature of 1040-1150° C. for 30-60 min; wherein a cooling is performed in a water-cooled mode; a grain size is required to be finer than 6 grade and a face hardness of a sample is 32˜37 HRC; wherein the online heat treatment includes the following steps: rolling 53Cr21Mn9Ni4N continuous casting billets through a high speed mill, wherein a heating temperature of rolling meets requirements described in the step 5, the wire rod is performed with the online heat treatment after spinning, and put into water for cooling, wherein the grain size is required to be finer than 8 grade, and the face hardness of the sample is 31˜36 HRC. 